1. Field of the Invention
This invention relates generally to snap action thermal actuators having narrow temperature differentials that are particularly useful for controlling the operation of the electrical contacts of a thermally actuated switch.
2. Prior Art
Thermal actuators for operating the contacts of a thermal switch are known. Typically, such actuators employ a bimetallic member, such as, for example, a disc, that has a high temperature stable state and a low temperature stable state that snaps with a snap action from the low temperature stable state to the high temperature stable state upon heating and returns to the low temperature stable state upon cooling. In such devices, the bimetallic member snaps to its high temperature stable state at a predetermined temperature known as the upper set point and returns to its low temperature stable state at a lower temperature known as the lower set point. The temperature difference between the upper and lower set points is known as the temperature differential. The temperature differential of an actuator may be adjusted to suit the desired application by adjusting the geometry of the bimetallic disc.
When operating the contacts of a thermal switch it is desirable to have a strong snap action to insure a rapid opening and closing of the contacts in order to reduce arcing and contact resistance to thereby prolong the life of the contact. A strong snap action is not difficult to achieve with an actuator employing a bimetallic member as long as the temperature differential of the member is relatively high. However, when the temperature differential is reduced, the quality of the snap action is also reduced and the contacts will be opened and closed more slowly.
One attempt to provide a quick acting temperature controlled electric switch is illustrated in U.S. Pat. No. 1,918,491. The device disclosed in the '491 patent utilizes two bimetallic discs carrying opposed contacts. The two discs have different thermal characteristics and cooperate to maintain pressure on the contacts as the opening temperature is approached. However, the device illustrated in the '491 patent will still have a relatively sluggish snap action when discs having a low temperature differential are used. Also the design is relatively complex, and cannot readily control externally mounted contacts. Consequently, the design cannot easily be integrated into existing switch designs.